Dynamics of the Transition corona Sophie Masson Space Weather Laboratory - NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt MD 3 [email protected] 1 0 and 2 n Patrick McCauley a J 4 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA ] R and S . Leon Golub h p - Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA o r t s and a [ Katharine K. Reeves 1 v 0 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 4 7 and 0 . 1 Ed DeLuca 0 3 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA : v i X ABSTRACT r a Magnetic reconnection between open and closed magnetic field in the corona is believed to play a crucial role in the corona / heliosphere coupling. At large scale, the exchange of open /closed connectivity is expected to occur in pseudo- streamer structures. However, there is neither clear observational evidence of how such coupling occurs in pseudo-streamers, nor evidence for how the magnetic reconnection evolves. Using a newly-developed technique, we enhance the off-limb magnetic fine structures observed with AIA and identify a pseudo-streamer-like feature located – 2 – close to the northern coronal hole. We first verify that the magnetic topology related to the observation is a null-point topology surrounded by open magnetic field of the coronal hole. We then compare the evolution of the observed pseudo- streamer fine structure with the location of strong currents, i.e. in the region of energy dissipation, with the dynamics of the magnetic field resulting from the interchange reconnection obtained in the fully 3D MHD simulation. The morphological and dynamical similarities between the pseudo-streamer observations and the results from the simulation strongly suggest that the evolu- tion of the pseudo-streamer is caused by interchange reconnection in a null-point topology that is embedded in Quasi-Separatrix layers. Besides identifying the mechanism at work in the large scale coupling between open and closed field, our results highlight that the interchange reconnection in pseudo-streamers is a grad- ual physical process and differs from the impulsive reconnection of the solar-jet model. Subject headings: methods: observational —methods: numerical — magnetohy- drodynamics — Sun: magnetic topology — Sun: corona 1. Introduction Null points are common features in the solar atmosphere (Schrijver & Title 2002), and many flares have been associated with such topology (D´emoulin et al. 1994; Mandrini et al. 1995; Fletcher et al. 2001; Masson et al. 2009; Des Jardins et al. 2009; Wang & Liu 2012). A null point forms when a single dipole emerges in one polarity of a dipolar active region (Antiochos 1998; Pariat et al. 2009a; To¨r¨ok et al. 2009; Edmondson et al. 2009; Masson et al. 2012), dividing the corona into distinct connectivity domains that are delimited by separatrices. Magnetic fields lines going through a three dimensional (3D) null point are of two types (Parnell et al. 1996; Lau & Finn 1990; Longcope 2005), fan and spine. The majority of the field lines form a surface, the fan, which has a domelike shape and generates a two-flux system with distinct connectivity. Two singular field lines, the two spines, also originate from the null, each belonging to one connectivity domain. For a dipole embedded in an open magnetic field, the fan surface separates the closed connectivity domain, confined below the fan, from the surrounding open magnetic flux, belonging to the outer connectivity domain. In this situation, the inner spine is rooted below the fan surface, and the outer spine is open into the corona (top left panel on figure 1). This null-point topology with an open outer spine defines the transition region between the open and the closed magnetic field in the corona. – 3 – Null-point topologies are identified in small-scale solar jets (Moreno-Insertis et al. 2008; Pariatetal.2009a;T¨oro¨ketal.2009)andlarge-scalepseudo-streamers(Wangetal.2007b,a). Solar jets are usually observed inside polar coronal holes (Shibata et al. 1992) and result from the emergence of a small scale dipole into an open magnetic field region. Conversely, pseudo-streamers consist of a larger-scale configuration and are associated with active re- gions. According to the multipolar property of the photospheric magnetic field, one also expects a null-point topology with the open spine to appear frequently at the border of the coronal hole boundary (Antiochos et al. 2007). Magnetic reconnection at a 3D-coronal null-point has been studied with numerical sim- ulations (Rickard & Titov 1996; Galsgaard et al. 2003; Masson et al. 2009; Pariat et al. 2009a; Galsgaard & Pontin 2011) and observations (Aulanier et al. 2000; Fletcher et al. 2001; Manoharan & Kundu 2003; Wang & Liu 2012). When magnetic reconnection takes place at a null-point embedded in an open magnetic field, it induces a connectivity exchange. Field lines initially closed below the fan reconnect at the null point with the open field and jump to the open connectivity domain outside of the fan surface. Meanwhile, the initially open field lines reconnect at the null point and close down below the fan. This process is known as interchange reconnection (Antiochos et al. 2002; Edmondson et al. 2009; Pariat et al. 2009a). In a recent numerical study, Masson et al. (2012) developed a model for magnetic reconnection at a 3D null-point. The model and the results are summarized in § 2. Using a newdevelopedtechniquetoenhancethecontrastofoff-limbfeaturesinAtmosphericImaging Assembly (AIA; Lemen et al. (2012)) data, we identify a set of observations showing the evolution of a pseudo-streamer-like structure located at the border of a northern coronal hole (§ 3). Comparing the numerical results and the observations, we conjecture in § 4 that the interchange reconnection model developed in Masson et al. (2012) may explain the dynamics of the AIA observations. 2. Tri-dimensional MHD simulation 2.1. Numerical model In order to study the dynamics of interchange reconnection, Masson et al. (2012), here- after M12, performed a tridimensional MHD simulation of a null point embedded in an open diverging magnetic field. They used the explicit 3D MHD code Observationally-driven High- order scheme Magnetohydrodynamic (OHM) code, developed by Aulanier et al. (2005). M12 used the enhanced version of the code containing the gravity finite-β effects. The OHM code – 4 – solves the non-conservative MHD equations in cartesian coordinates on a non uniform mesh in the 3 directions using high precision algorithm in time and space (see Masson et al. (2012) for details). The numerical model of M12 focuses on the magnetic field dynamics, but does not treat properly the plasma response. In the continuity equation, a nonphysical explicit diffusive term, which smooths the gradients, has been added. This term helps to stabilize the compu- tation by avoiding cavitation or shock formation. In addition, the energy equation is reduced to the adiabatic temperature equation, to which a nonphysical explicit diffusive term has also been added for gradient smoothing. Therefore, the plasma is only heated by adiabatic effects and cools down via the diffusive term. Any other heating or cooling processes (e. g. Joule dissipation or radiation losses) are not included in the simulation. The initial magnetic configuration is built by positioning two magnetic monopoles below the photosphere in a region of open diverging magnetic field. This configuration emulates a dipolar active region embedded in an open diverging magnetic field, e. g. a coronal hole. This produces a 3D asymmetric null-point topology with an open outer spine (Figure 1). The initial atmosphere is stratified, obeying hydrostatic equilibrium, and the temperature, gravity and density profiles are adjusted to obtain a solar-like regime: β is lower than 1 in the corona except at the null-point where β becomes greater than 1. In order to emulate the physics of the solar corona, the top and side boundaries of the numerical box are open, whereas the photospheric boundary is reflecting and line-tied. The system is forced by applying a sub-Alvf´enic photospheric velocity flow, which moves the positive polarity toward negative y in the area of x ∈ [−7,3] and y ∈ [10,−30], where a part of the fan is rooted. 2.2. Interchange and slipping reconnection The photospheric flow shears one part of the magnetic field lines rooted below the fan, leadingtoabulgingoffieldlinesthatcompressestheseparatricesandaregionofover-density forms along the separatrices (Figure 1 and figure 2 in Masson et al. (2012)). Moreover, it induces a compression of the null-point and it misaligns the inner and the outer spine. The shearing of the spines increases the region of weak magnetic field around the null point where an intense current sheet develops (Low 1987; Aly 1990; Lau 1993; Rickard & Titov 1996; Galsgaard et al. 2003; Pariat et al. 2009b). Therefore, magnetic reconnection is expected to occur at the null point in order to bring the spines back into alignment (Antiochos et al. 2002). – 5 – In order to determine the dynamics of magnetic reconnection, Masson et al. (2012) plotted field lines from footpoints fixed in the advected positive polarity, integrated them up to their conjugate footpoints and followed the evolution of the connectivities during the simulation. Thetoprowonfigure1showstheevolutionat3differenttimesofthesimulation. Initially, 3 groups of colored field lines have been plotted closed below the fan, and 4 groups of colored field lines open into the corona. Following the connectivities of the plotted field lines, M12 showed that magnetic recon- nection occurs at the null-point through interchange reconnection. The initially closed and open magnetic field lines exchange their connectivity and respectively open into the corona and close down below the fan surface. However, as the authors pointed out, the reconnected open field lines do not remain close to the outer spine, but display an apparent slipping motion that leads to the extension of the reconnected open flux. These dynamics result from the slip-running and slipping reconnection regime across quasi-separatrix layers (QSLs) that surround the fan-spine topology (see online material, animation1). A quasi-separatrix layer defines a region of strong gradients of connectivity (Priest & D´emoulin 1995), where finite- width thin current sheets develop and field lines exchange their connectivity continuously between neighboring field lines (Priest et al. 2003; D´emoulin 2006; Aulanier et al. 2006). This continuous reconnection of the open field lines across the QSL leads to the formation of an extended open reconnected magnetic flux. The shape of this squashed open flux is determined by the spatial distribution of the QSLs surrounding the open outer spine (see Masson et al. 2012). 3. Observations We examine a streamer structure on the west limb, at the border of a northern coronal hole. The observations reported here were taken on January 19, 2012 by the AIA instrument aboard the Solar Dynamics Observatory (SDO; Pesnell et al. 2012). AIA records full-sun images in seven extreme ultraviolet channels with a 12 s cadence and a resolution of 0.6(cid:48)(cid:48) per pixel. Of these, the 171 ˚A band is most suited to our model comparison because it best resolves the fine magnetic structures in the corona. This channel is dominated by Fe IX, with a characteristic temperature of 105.8 K. To further enhance fine structure, we have processed the observations using a radial filter. This begins by summing the off-limb component of several images to increase signal- to-noise. The corona beyond the disk is then divided into concentric rings, each of which is scaled as a function of its radius, average brightness, and intensity relative to neighboring rings. As such, flux is not conserved; the brightness of each pixel corresponds only to its – 6 – intensity relative to other pixels of the same radius. Additional information and source code is provided in the SolarSoft routine <aia rfilter>. For this application, we have used 10-image sums, yielding resultant images with total integration times of ∼20 s over 2 min periods. Figure 2 displays a set of six images between 11 : 00 UT and 23 : 00 UT, and a corresponding movie is available in the online material. While the heated plasma is constrained by the magnetic field, the white strands observed in AIA indicate explicitly the geometry and the directivity of the magnetic field. On Figure 2, we observe that some of the strands are closed, forming two lobes, and some other strands are open and surround the two closed lobes. In addition, the general pattern of the event does not change drastically, but the fine strand structures are highly dynamic (see the online material). The morphology and the spatial evolution of the closed and open field lines look like it could be the signature of interchange reconnection at the null point. 4. Can the AIA observations be explained by interchange reconnection in the corona ? 4.1. Magnetic configuration The event described above occurs at high latitude, and according to the magnetogram, it is related to a decayed dipolar active region that has migrated toward the pole. Such a multipolar photospheric distribution of the magnetic field is conducive to a null point formation with an open outer spine (Antiochos et al. 2007). In order to determine the magnetic topology related to this observation, we performed a potential field extrapolation of the magnetic field measured by HMI (Schou et al. 2012) onboard SDO using the Potential Field Source Surface (PFSS) model (Schrijver & De Rosa 2003). Even though the PFSS model cannot reproduce the exact magnetic configuration of the corona, it still gives an approximation of the global magnetic topology (Brown & Priest 2001). Figure 3 shows the magnetic configuration resulting from the potential field extrapola- tion, where the white field lines indicate the closed magnetic field and the pink field lines correspond to the open magnetic field. The anemone shape displayed by the closed white field lines (right panel on figure 3), surrounded by open pink field lines, corresponds to the magnetic configuration associated with a null-point topology with an open outer spine. – 7 – 4.2. A proxy of the heated plasma location In the M12 simulation, the density and temperature evolution do not reflect a correct plasma response (see Sect. 2.1). Therefore, the density and temperature cannot be used to generate synthetic images of the emission (e.g. Reeves et al. 2010) for comparison with the AIA observations. However, we propose to use the current density distribution to determine the regions where plasma emission should be observed. In MHD formalism, the energy dissipation and transfer occur in regions of intense and thin current sheets. Thus, the current sheet location will indicate the regions where heated plasma may be found. In addition, the current density is also localized where the Joule heating, ∝ ηj2 can contribute. Finally, the current intensity will also be stronger in the magnetic field compression region, e. g. along the stressed separatrices, indicating the location of the over-dense and heated plasma. The strong currents are expected to be localized around the null point and along the separatrices, but also along the Quasi-Separatrix layers. In M12, the currents localized in the QSLsurroundingtheouterspineappearmuchweakerthanthecurrentalongtheseparatrices and the null point (see Figure 4 in Masson et al. (2012)). This difference of intensity partially results from the decrease of the magnetic field magnitude with the altitude. But it is also a direct consequence of the spatial resolution of the numerical grid. Given that current density is derived from the magnetic field, the intensity of the current sheet is more diffuse when the grid is coarser, as it is above the null point. However, we expect that strong and thin current sheets develop inside the QSLs (Aulanier et al. 2006). In order to increase the contrast of the region where strong currents should form, we used α = j.b/b2 instead of the current density. Indeed, this quantity is less dependent on the magnetic field magnitude but takes into account the orientation of the currents with respect to the magnetic field. Therefore, α highlights current sheets in the regions of weak magnetic field and with strong magnetic gradient. In order to be consistent with the AIA observations, integrated along the line-of-sight, we integrate the α parameter along the y axis from the boundary y = −30 to the (x,z)-plane at y = 0. This integration range avoids the part of the QSL located between y = 0 and y = 30, which results from the symmetry of the system (see Masson et al. 2012) and should not exist in a real magnetic configuration. 4.3. Pattern and dynamics of the event : observations vs simulation The bottom row on figure 1 shows the 2D-maps of the integrated value of α at three different times. According to the dynamics of the magnetic field and the separatrices during the simulation (top row on figure 1), we identify the magnetic structures associated with – 8 – the bright regions of α (refer to labels on figure 1). The brightest region indicated on this 2D α-map corresponds to the current sheet located around the null point. The fan separatrix corresponds to the arc-shaped white structure, which delimits the open and the closed magnetic field. The inner spine can be identified by the white structure highlighted on the right panel on Fig. 1. The extended white-halo structure covering the area between x ∈ [−15;−5] and y ∈ [0;20] is co-spatial with the reconnected open field lines belonging to the QSL. The outer spine borders the left edge of this white-halo structure, emanates from the null point and reaches the top of the numerical box. Even though the intensity of the integrated-α does not reproduce the emission, it shows the location where heated plasma may be found. Comparing the AIA images and the syn- thetic α-map, we notice clear similarities between both. The emitting regions in AIA show the same patterns as the structures identified in the 2D-α map. We identify the thin and bright region surrounding the null, the arc-shape white structures related to the fan separa- trix and the bright lines corresponding to the inner and outer spines. The white structure above the fan is compounded of multiple strands linking the photosphere and the outer boundary of the image, and likely corresponds to the QSL field lines (refer to the label on figure 2). Previous studies already show an excellent association between the current density from simulations and the hot plasma emission at different wavelengths (e. g. Delann´ee et al. 2008; Aulanier et al. 2010; Savcheva et al. 2012). Thus, the morphological similarities between the observations and the simulation highlight that the emission regions in the AIA images are defined by the topological objects, the separatrices and QSLs. Note that the differential method used to produce the images of AIA allows us to visualize the QSL structures in the corona in the AIA images Since the event is associated with a null-point magnetic topology (§ 4.1) and considering that the topology is associated with the AIA emission, the interchange/slipping reconnection may be the mechanism explaining the AIA observations. The AIA observations show a 2D projected evolution of a purely 3D phenomenon. We notice that some field lines initially closed, grow and get closer to the fan before disappearing (see animation 2). In the meantime there are also loops on the upper side of the null point that collapse in to it (animation 2 and 3). Around 14:00 -15:00 UT there are loops underneath the null that shrink down, moving in the opposite sense of the loop pointed to in animation 2. Combined with line-of-sight effects, these field lines motions are consistent with the dynamics of M12 simulation. The initially closed field lines below the fan surface bulge and slip toward the null point, then reconnect at the null point and open in the corona. Furthermore, the open flux tubes moves away from the null-point area and show – 9 – an apparent slipping motion (see animation 3) and may therefore correspond to the open field lines continuously reconnecting across the QSL. We also notice that the bright structure related to the null point on the observations (top middle and right panel on Fig. 2) and on the α-map (bottom row on Fig. 1) becomes thinner and more intense as time goes on. The dynamics of the fine structure observed with AIA are consistent with the dynamics of the magnetic field derived from the simulation (see animation 1). Combined with the morphological similarities between the α-map and the plasma emission, and the null-point magnetic topology obtained from the extrapolation, we are confident that the evolution of the AIA observation on Jan 19, 2012 results from magnetic reconnection in a null-point topology, combining interchange and slipping reconnection. 5. General statements In this study, we show that the dynamics of the pseudo-streamer like structure at the coronal hole boundary, on January 19, 2012, results from the interchange/slipping recon- nection regime (Masson et al. 2012). The interchange reconnection is believed to play a crucial role for the solar-heliospheric coupling, specially because it allows energy, mass and waves transfer from the closed corona into the heliosphere. According to our results, major statements can be done : • Theinterchange/slippingreconnectionmodecouplingtheopenandtheclosedmagnetic field is not an explosive mechanism like the twisting-driven interchange reconnection mode of the solar polar jet (Pariat et al. 2009a). This suggests that the interchange solar jet model can not be applied to model reconnection in the large scale pseudo- streamers. • According to the S-web Model (Antiochos et al. 2011; Titov et al. 2011), the global topology of the solar coronal magnetic field should implies that such interchange / slippingreconnectiondynamicsshouldoccureverywherealongacoronalholeboundary. The event presented in this study is not an isolated case. In a forthcoming study, we will investigate the morphological and dynamical characteristics of other events from AIA data set and determine the effect of the closed/open coupling on the solar-heliospheric physics. – 10 – Fig. 1.— The top row: Evolution of the magnetic field lines during the interchange reconnec- tion process. The greyscale code at the bottom boundary represents the vertical component of the magnetic field, white and black color show respectively the positive and negative val- ues. The dark blue field lines denote the fan and spine separatrix field lines and the group of colored field lines (pink, red, yellow, light blue) displays the open and the closed field lines that reconnect at the null point and exchange their connectivity. Bottom row : temporal series of the 2D-map of the integrated value of α, colored by greyscale .